A spark plug is used in a combustion apparatus such as internal combustion engine (sometimes simply referred to as “engine”) for ignition of an air-fuel mixture in a combustion chamber of the combustion apparatus. In general, the spark plug includes an insulator having an axial hole in an axis direction of the spark plug, a center electrode inserted in a front end side of the axial hole, a metal shell arranged around an outer circumferential surface of the insulator and a ground electrode joined to a front end portion of the metal shell so as to define a spark discharge gap between the center electrode and the ground electrode. A leg portion is formed on a front end side of the insulator such that an annular clearance is left between an outer circumferential surface of the leg portion and an inner circumferential surface of the metal shell. Further, a step portion and an engagement portion are formed on the inner circumferential surface of the metal shell and the outer circumferential surface of the insulator, respectively, such that the insulator is held in the metal shell by engagement of the engagement portion on the step portion via a metal plate packing (see, for example, Japanese Laid-Open Patent Publication No. H10-289777).
There is a possibility that carbon substance is generated by incomplete combustion of the air-fuel mixture in the combustion chamber and deposited on the surface of the leg portion. With the progress of such carbon deposition, the surface of the leg portion may be covered and fouled with the carbon deposit. In this case, the spark plug fails to cause a normal spark discharge in the spark discharge gap but can cause an air discharge between the insulator and the metal shell in the inner side of the clearance by the flow of electric current from the center electrode to the metal shell through the carbon deposit.
In recent years, the metal shell has been reduced in diameter for size reduction (diameter reduction) of the spark plug. However, the diameter reduction of the metal shell leads to a decrease in the size of the clearance between the inner circumferential surface of the metal shell and the outer circumferential surface of the leg portion in a direction perpendicular to the axis direction. As a result, the occurrence of an air discharge between the insulator and the metal shell due to the carbon deposit becomes of more concern.
It is conceivable to elongate the leg portion in order to achieve good fouling resistance even in the case where the metal shell is relatively small in diameter. In this technique, the entry of carbon substance into the inner side of the clearance, in which the occurrence of an air discharge is of particular concern, can be more assuredly prevented by the elongated leg portion for improvement in fouling resistance as the size of the clearance in the direction perpendicular to the axis direction is made relatively small.
When the leg portion is elongated, however, the front end part (leg portion) of the insulator is readily overheated during operation of the internal combustion engine or the like. It is thus likely that pre-ignition will occur by the action of the overheated front end part (leg portion) of the insulator as a heat source. As the front end part (leg portion) of the insulator has more tendency to be overheated with the recent improvement of engine output performance, the occurrence of pre-ignition becomes of more concern. For these reasons, it has been demanded to improve the fouling resistance of the spark plug without elongating the leg portion for prevention of overheating of the leg portion (i.e. for prevention of pre-ignition).
The present invention has been established in view of the above circumstances. An advantage of the present invention is a spark plug capable of achieving very good fouling resistance even when it is of particular concern that an air discharge occurs between an insulator and a metal shell by diameter reduction of the metal shell.